Synchronous motor control system



May 17, 1949. s. c. EWING 2,470,478

SYNCHRONOUS MOTOR CONTROL SYSTEM Filed June 11, 1948 Inventor Samuel CEw'mg,

His ALtOrney.

Patented May 1 7, 1949 SYNCHRONOUS 'MOTOR CONTROL SYSTEM aSamuel ce-Ewing scotia, N. Y. ,'assig-nor to General- Electric.Company, a: corporation of- New York -Application' June 11, 1948, SerialNo. 32,287

1- ".2 Claims. (Cl. '318-176) This invention relates to COIltIOLSYStGIIlS'; more 1 particularly to controlzsystem'slior synchronous "motoraand it has for an nbjectxthe provision of a simple, reliable andimproVedecontrol system of this character.

Another object of the-inventionisanr improved u control system for applying field to-a: synchronous machine at a -p'redetermined' subsy'n'chronous w speed of themachine.

A still further object of the invention is :the provision of a synchronous motorcontrolzsystem I iii-which thenumber of controldevicesrfor .carrying out the control operations isxreduced to a minimum.

In-carrying the inventioninto' ei'iectv in:one form thereof, a' 'contactor is proVided':for*?'controlling the connection of th'e' field winding of a synchronous-motor toralsource of 11-6. supply wand a line contactor is pi OVide'dforrconnecting the armaturewin'ding to alsource of A.-C. supply. A field relay is providedfo-r controlling the energization of the 'fieldcontactori Ithastwo coils mounted on-its core structure'. Onefcoilis a closing coil and the other is-energized iromathe slip frequency voltage induced in the field .winding during-operation of the motor atsubsy'nchronous' speed and controlsthe dropout-operation of the field relay after the closing coil: has :been deenergizedfi 'The' 'deenerg-izatior-i of the-closing 'coil' to transfer: the -control'of the'sy nchronizing sequence tothe drop' out coil isefiected by means of' an auxiliary relay in response: to closing of "the line contactor. At a' predeterminedspeed of the motor nearxsynchronism;the drop-tout coil control permits the field relay. to drop. outand energize thefield contactora'rThefieldlrelay is if interlocked" with the": auxiliary .relay to lprovide :for deenergization ofv therauxiliary relay in re- W-sponseto drop-out ofwthfield-crelay 'l The r; auxiliary relay has 1 time :delay dropx-out, andiidurs period it! prevents reenergization s of-theclosingicoil ofthefield relay.

For abetter and moremomplete understanding of: the invention, :reierence should.:now. be; had

to the following specification-and toa-thenaccomof a synchronous motor l is arranged tobe con- .mected by :means of 'ai'cnntactor 2 to a :suitable 2x? source of alternatinglvoltage' whichxis-represented :zrbyrtheitlrreesupplyconductors 3,:- 4;and 5. w'lhe irfieldewindings lb of thermotor isiarranged to be arconriected by:imeans of f8; suitablaswitching de source" of. substantially .cOnStantrtdirect voltage which is represented by the two supplyvlines I and la.

For the purpose ofuopening...andclosingathe line switch contacto'r 2-,;:suitable fifstartffz. and stop switching devices and e are provided. The start device 8 is illustrated as a. push button type switch having normally. open contacts and the stop device :9 is'illustrated as a'a1push'xhutton type switch having normally :closed. :contacts. The normally closed contacts of thei.start"switch 8 are connected in the operatingxcoil. circuit of an auxiliary :relay I0, andztheinormallysclosed contacts of the stop switch 9 are connected in theoperating .coil circuits of..the line. contactor 2 and the auxiliary relay l0.

A discharge resistor H is providedforlthefielcl winding. It is connected. across the winding by the normally closed contacts 61) oithefield' contactor 6. Thenormallyopen contacts 6.0:. and 6d serve tovconnect .theifieldzwindmg lib to the source I, lawhenthe field contactor is picked up.

A suitable speed responsive means is provided for controllingthe energization ofwthe' operating coil- 60; of thei-fieldficontactor when. the motor attains substantially synchronous speedi; .This

. means is illustrated as comprising avlre'lay E2 of a which one of its operating coils 12a is connected v the field relay to'close itsnormally:openzcontacts [20 to connect the-winding IZawacrOss: the resistor II. The rectifier I3 is poled to rectify. the negative; half cycles of the:alternating-voltage across the resistor H. With:thisearrangement the induced :currentwwhichrfiowsin; the" circuit of the operating coil l 2a1i-maintainstherrelay ..;picked up' and-.itscontacts lZcZropent until the xmotor attains a speedzwhich is :a'fewnpereicent below synchronous speed, e. g. 51 to ':5 zpericent slip. At this speed the frequency of the induced voltage is so low that theportion of-each slip cycle during which the-rectifier permits substana-t-iallyv no current to fiowthrough'thearelay-dropout control winding [2a is sufficiently long to allow the relay to become demagnetized and to drop out and close its normally closed contacts l2d.

A relay l4 having instantaneous pick up and time delay drop-out cooperates with the field relay l2 to control the sequence of steps in the synchronizing operation. In this connection it responds to the closing of the line contactor 2 to open the circuit of the closing coil l2b of the field relay IE to transfer control of the remainder of the synchronizing operation to the dropout control coil l2a. Also it responds to the drop-out of the field relay l2 to prevent its reenergization for a predetermined interval of time after the drop-out.

For the purpose of deenergizing the field winding lb in the event the motor pulls out of synchronism, a pull-out relay I5 is provided. This pull-out relay is preferably a power factor relay.

It has a current coil l5a which is connected t the secondary winding of a current transformer [6 of which the primary winding is energized by the current flowing in one of the phases of the armature of the motor. It also has a voltage coil l5b which is connected through a resistor ii, the contacts lllb of the auxiliary relay l6, and the contacts of the start switch 8 and the stop switch 9 across the remaining two phases of the A.-C. supply. The relay I5 is provided with normally open contacts which are connected in the circuit of the operating coil 6a of the field contactor.

The relay is provided with an E-shaped core structure and with an armature member which is biased to the drop-out position by means of a spring I56. The voltage coil and the current coil are both mounted on the same leg of the E-shaped core structure and preferably on the center leg. If the magnetomotive forces of the two coils are in phase and tending to send flux in the same direction, and further if the voltage and current being supplied to the motor have sufficient magnitudes, the relay will pick up and close its contacts. As the phase angle between these magnetomotive forces changes owing a change in the phase angle between the current in the coil l5a and the current in the coil Eat the magnetomotive forces tend to buck each other and the relay will trip at a phase angle of the motor voltage and current which is indicative of the motor having pulled out of synchronism.

The relay I0 is provided with normally open contacts lflo which, when closed, short-circuit a large portion of the resistor ll in circuit with the voltage coil of the pull-out relay. The portion of the resistor i! that is short-circuited may be varied by means of a selector switch llc. Preferably a portion is selected which will result in twice normal current in the coil circuit when the selected portion is short-circuited. This wil re sult in the voltage coil overwhelming the current coil and picking up the relay irrespective of the power factor angle between the voltage and current of the motor.

With the foregoing understanding of elements and their organization, the operation of the system will readily be understood from the following detailed description.

The starting operation is initiated by pressing the start switch 8 to complete an energizing circuit for the operating coil l 0a of the auxiliary relay l0. In response to energization, relay iii picks up and closes its normally open contacts lllb, I00 and llld. Contacts 100 in closing shortcircuit two sections of the resistor ll. Contacts lllb and Mid complete an energizing circuit for the operating coil 2a of the line contactor. This circuit is traced from the A.-C. supply conductor 3, through the stop and start switches Q and 3, contacts lllb and llld and operating coil 2a to conductor 6. Responsively to energization, the line contactor closes its main contacts to connect the armature winding of the motor to the A.-C. supply conductors 3, 4 and 5. It also closes its interlock contacts 21) and 20. In closing, contacts complete a sealing-in circuit for the line-com tactor in parallel with the contacts 5% and the contacts of the start switch 8 which may now be released. A sealing-in circuit is also provided for operating coil of auxiliary relay it which extends from conductor 3 through stop switch 9, contacts 20 of the line switch, contacts 5th of the auxiliary relay, contacts 6e of the field contactor and coil lila of the auxiliary relay to conductor i. The closing of interlock contacts oi the line switch completes an energizing circuit for the voltage coil l5b of the pull-out relay it. It is traced from supply conductor l through coil use, the right-hand section of resistor ll, contacts ltc, contacts 20 of the line switch and contacts of the stop switch 9 to supply conductor 3. Since a large portion of resistor ll is short-circuited, the voltage coil l5b is so strongly energized that it causes relay Hi to pick up and close its contacts I50.

In closing, contacts 219 of the line switch complete an energizing circuit for the closing, coil of the field relay which extends from the positive conductor l through contacts 2b, 6) and i l-b through coil lZb to negative conductor la. The field relay l2 picks up in response to energization and closes its normally open contacts I20 and and opens its normally closed contacts l2d.

Contacts Me, in closing, complete an energizing circuit for the drop-out control coil lZa across a portion of the discharge resistor l I.

Since the line contactor 2 is closed, the stator winding la of the motor is connected to the A.-C. supply source, the motor accelerates as a squirrel cage motor, and an alternating voltage of slip frequency is induced in the field winding lb and appears across the discharge resistor l l to which the field winding is connected through normally closed contacts 5b of the field contactor. Owing to the presence of the rectifier l3 in the circuit of coil l2a, a half wave current flows in the coil l2a.

In their closed position, the contacts lie complete an energizing circuit for the operating coil I la of the time delay auxiliary relay it which picks up without time delay in response to energization to open its normally closed contacts Mo and to close its normally open contacts Hlc and Mid. In opening, contacts l lb interrupt the energizing circuit of the closing coil of the field relay which, however, remains held in as a result of the energization of the drop-out control coil 52a by the rectified current impulses supplied from the field circuit. Thus control of the remainder of the synchronizing operation is transferred frcm the closing coil to the drop-out control coil.

Contacts [40, in closing, complete a circuit in parallel with interlock contacts 6g of the field contactor and contacts I of the pull-out relay. Similarly contacts lad complete a circuit in parallel with contacts 6e.

As the motor continues to accelerate, the slip frequency of the half wave voltage supplied to the drop-out control coil decreases from cycles (for a 60 cycle machine) at the start to a low frequency, e. g. the frequency corresponding to 1 7&4750378 twe perecent slip. At' this rrequency *theiint'erval etween: successive current impulses is- 'srr ig reat hat the drop outcoir "I 2a becomes deenergized "for a time sufiicient to-eriable theirelay to drop outand close' its morm'ally closed contacts lid and open its" normally ope-n c's'ontacts Itc -arid I 22. -G-ontacts I 20- in-"opening interrupt' the-circuit of l the drop-out coil-l 2a of --tl:1'e' field relay. -o-ntacts I2e; in-opening, interrupt :the circuit -of+ comm 4a of the'timedelay' auxiliaryrelaywhichbegins to time out in response'to-deenergization.

The-closing of contacts l 2d completes. an enercontactor fi'f This circuit is traced from the supply conductor 4, through coil 6a, closed con-x.

tacts I2d, contacts-I40 of the time delay relay which has not yet timed out, interlock contacts 20 of the line switch, contacts of stop switch to conductor 3. Responsively to energization, the field contactor 6 picks up and opens its normally closed contacts and closes its normally open contacts. Contacts 622 in opening disconnect the field winding Ib from the discharge resistor II and contacts 60 and Edin closing connect the field winding lb to the excitation source supply conductors I and 7a. Contacts 69, in their closed position, complete a sealing-in circuit for the coil 6a through contacts Ic of the pull-out relay I5 to maintain the coil 6a of the field contactor 6 energized after the auxiliary relay I4 times out and opens its contacts M0.

The application of field to the motor usually results in the motor pulling into synchronism. Immediately following the application of field there is a possibility of momentary surges and unstable operation in which the power factor changes abruptly and erratically and in which the pull-out relay I5 would remove field from the motor if it were permitted to do so. It is desirable not to remove field immediately after application of field because instabilities in the switching operation will usually disappear and the motor will settle down to operation at unity or leading power factor within a short period.

At the end of this period, the time delay auxiliary relay I4 has timed out and closed its contacts I41) and opened its contacts I40 and Mid. Contacts Md in opening interrupt the energizing circuit for the operating coil of auxiliary relay III, which drops out to open all its contacts. Contacts H10, in opening, insert all of resistor IT in circuit with the voltag coil of the pull-out relay.

If stable operation has resulted, the opening of contacts I 40 of time delay relay does not drop out the field contactor 6 which remains picked up through the contacts I50 of the pull-out relay I5. However, if stable operation has not resulted, or if at any subsequent time in its operation the motor pulls out of synchronism, the power factor of the voltage and current supplied to the motor becomes sufficiently less than unity to permit the power factor relay I5 to drop out and open its contacts I50 thereby to interrupt the energizing circuit for the operating coil 61; of the field contactor. Responsively to deenergization the field contactor 6 drops out and disconnects the field Winding Ib from the excitation source and reconnects it to the discharge resistor II. Interlock contacts 6h of the field contactor, in opening, interrupt the operating coil circuit of the line contactor which drops out in response to deenergization to disconnect the stator winding of the motor from the A.-C. source thereby to have the system in a reset condition for starting in response to depression of the start switch.

6 .Ifiiautomatic' -resynchronizing: operation desired,- the-;iumper's I8 and I fi'shown inzdotted v lines may-- be p'rovidedZ :Thejumper I8.pa'rallels.1the 'eontacts' 6h of the field-contactor -5- -and the jumper"I fi-parallels the contac'ts I 01') of ithe auxilia'ryrelay I0.

Glorrsequently when the field contactor 'drops -outand opens contacts 8h',-the' operating 'coil circuit' of the line contactor is not interrupted l'fi but' remains cornpleted through-the jumper I8. -In-" its dropped out position I the" fieldcontactor close's its interlock 'contact's 6e to c'omplete an energizing 'circuit forthe operating coil a of =therauxiliary relay which extends fromthe left hand-contact or the-start switch through interlock riontacts 2c of --tl'ie*lineswitch; jumper I9, 'contacts 8e of-the field "contactor,- and operating coil IOa of the auxiliary relay which responds and closes its contacts. From this point on the operation is the same as described in the foregoing following the energization of the auxiliary relay II! in response to depression of the start switch 8.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle thereof has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the elements shown and described are merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

In combination, a synchronous motor having field and armature windings, a source of D.-C. excitation, means for connecting said field winding to said source comprising a field contactor provided with an operating coil, means for controlling said contactor comprising a field relay provided with a closing coil, a drop-out control coil, a contact for connecting said dropout coil to the field circuit of said motor and with a contact in the coil circuit of said contactor, a starting switching device, a source of alternating voltage, a line contactor responsive to operation of said switching device for connecting said armature winding to said alternating source and provided with an interlock contact for energizing said closing coil to cause said field relay to pick up and connect said drop-out control coil to said field circuit, an auxiliary relay responsive to operation of said field relay for deenergizing said closing coil thereby to transfer control of said field relay to said drop-out coil, a rectifier in circuit with said drop-out coil for supplying unidirectional current pulses to said drop-out coil to effect drop-out of said field relay in response to a predetermined duration of interval between successive impulses, and a normally closed contact on said field relay in the coil circuit of said field contactor for closing said field contactor to connect said field Winding to said D.-C. source in response to drop-out of said field relay.

2. In combination, a synchronous motor having an armature and a field winding, a source of D.-C. excitation, a field contactor provided with an operating coil and with contacts for connecting said field winding to said source, control means for said contactor comprising a field relay provided with a closing coil, a drop-out control coil, a contact for connecting said drop-out coil to the field circuit of said motor and with a normally closed contact in the coil circuit of said contactor, a source of alternating voltage, a starting switching device, a line contactor responsive to operation of said starting switching device for connecting said armature winding to said source of alternating voltage and provided with an interlock contact for completing an energizing circuit for said closing coil to cause said relay to connect said drop-out control coil to said field circuit, an auxiliary time delay relay responsive to said circuit completing operation of said field relay for deenergizing said closing coil thereby to transfer control of said field relay to said drop-out coil, a rectifier in circuit with said drop-out coil for supplying unidirectional current impulses to said drop-out coil during operation of said motor at subsynchronous speed thereby to effect drop-out of said field relay and closing of said field contactor to connect said field winding to said D.-C. source in response to a predetermined duration of interval between successive current impulses and a normally open contact on said field relay in the coil circuit of said auxiliary relay to effect drop-out of said auxiliary relay to close said normally closed contact to prepare an energizing circuit for said closing coil a predetermined interval of time after the closing operation of said field contactor.

SAMUEL C. EWING.

N 0 references cited. 

